This application pertains to the art of boosters and, more particularly, to boosters of the type commonly used in power braking systems for vehicles. The invention is particularly applicable to a combined hydraulic and vacuum booster. However, it will be appreciated that certain aspects of the present invention may be used in hydraulic boosters alone and in devices other than power brake systems for vehicles.
Hydraulic boosters commonly have an inlet port communicating with a cylinder on only one side of the piston, with hydraulic fluid flowing through a normally open valve in the piston when the booster is not energized. Flow of all the hydraulic fluid through the valve in the piston requires a relatively large valve, particularly when the hydraulic fluid is cold and has a high viscosity. A large valve through the piston provides poor controllability and the valve tends to oscillate. Minimizing oscillation of the large valve requires complicated balancing of the valve and maintenance of close tolerances. The piston bypass feature allows the use of a small valve yet permits adequate flow of cold fluid when booster is in passive position.
In hydraulic boosters of the type described, the valve itself does not operate to provide feedback forces to the brake operating means for providing an indication at the brake pedal of the force being produced by the hydraulic booster.